421 email error is typically a temporary routing issue on the receiving mail server's end. These should usually be resolved automatically, and you should receive a final bounce-back message if the message completely fails.

You usually would not receive a bounce-back message for this type of error. If your message is going to be delayed more than 24 hours, then sometimes a "delivery has been delayed" soft bounce will be returned to you, to let you know that the delivery has yet to complete succesfully.

Common variations of the 421 email error

  • 421 4.2.1 : (DYN:T1) http://postmaster.info.aol.com/errors/421dynt1.html
  • 421 4.3.1 Out of disk space for temp file
  • 421 4.3.2 All server ports are busy
  • 421 4.3.2 All server ports are busy
  • 421 4.4.2 mx.aol.com Error: timeout exceeded
  • 421 #4.4.5 Too many connections from your host.
  • 421 #4.4.5 Too many connections to this host.
  • 421 4.7.0 mx.aol.com Error: too many errors
  • 421 4.7.1 : (DYN:T1) http://postmaster.info.aol.com/errors/421dynt1.html
  • 421 4.7.1 POL-T2 http://postmaster.facebook.com/response_codes?ip= Connection refused
  • 421 4.7.1 POL-T2 http://postmaster.facebook.com/response_codes?ip= Connection refused

  • 421 Downstream server error
  • 421 Fork failed
  • 421 Greylisting enabled, please try later.
  • 421 LDAP verification agent temporarily unavailable: Connection failed
  • 421 mail.RemoteDomain.com out of connection slots
  • 421 mx.aol.com Service unavailable - try again later
  • 421 <recipient@RemoteDomain.com> Service not available - too busy
  • 421 Server busy, try again later - psmtp
  • 421 service temporarily unavailable
  • 421 Temporary error (#4.3.0)
  • 421 Too many concurrent SMTP connections; please try again later.
  • 421 Unexpected failure, please try later

Resolving a 421 email error

Because errors in the 421 series don't bounceback right away they would be considered a soft bounce. To better understand soft and hard bounces, and general email bounceback errors you can read why does email bounce, bounceback, or error?

Seeing a large amount of 421 email errors in your server's mail logs could be an indication that a lot of your mail users are having difficulties getting their e-mails delivered out successfully on the first attempt.

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n/a Points
2014-05-29 1:56 pm

I keep getting "too many simultaneous connections" for my AOL account. I have a multitude of phones, tablets, and computers that have my AOL account attached. Is there a limit to how many "connections" I can have? If so< what is my next step? If not. how to I remedy this?



9,521 Points
2014-05-29 3:59 pm
Hello Kady, and thanks for your comment.

Are you getting this error just when trying to send email to an AOL email account? This guide itself is on 421 errors, and you would typically see it in a bounce-back message after a message you sent to a AOL user failed to send. In this case you would want to try to send the message again at a later time as the AOL server could be temporarily overloaded.

If you are just getting this error in your mail client, and you are using your AOL account in multiple devices at once, you might be having an issue with IMAP usage on their server.

You might need to stagger the times your different devices check your mail. For instance, have your phone check every 5 minutes, tablets every 7 minutes, first computer every 9 minutes, and so on. You might also want to reduce the total number of messages stored in your main Inbox folder, and archive away messages you don't need to access often. Some email clients will tie up IMAP connections by trying to download lots of old message headers from the server for a message that's older than 30 days and you might not be reading anyways.

Hope that helps, please let us know if you still have any questions at all!

- Jacob

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